Bonding

What you'll learn

• How electron arrangements link to the Periodic Table.
• Why atoms form ionic bonds, covalent bonds and metallic bonds.
• How to draw dot-and-cross diagrams for simple ionic and covalent substances.
• How to compare ionic compounds, simple molecules, giant covalent structures, polymers and metals.

Start from atoms

An atom is the smallest particle of an element that can still be recognised as that element. It has a tiny central nucleus, containing protons and neutrons, with electrons arranged in energy levels, often called shells, around it.

Electrons are the particles involved in bonding. The nucleus does not change when atoms bond.

Atoms are extremely small: a typical atom is about 0.1 nanometres across, which is about $1\times 10^{-10}m$.

Electron shells and the Periodic Table

The electron arrangement of an atom shows how its electrons are arranged in shells. For the first 20 elements, the simple GCSE model fills shells as 2, then 8, then 8, then the next shell begins.

For example, sodium has atomic number 11. A neutral sodium atom has 11 electrons, arranged as 2,8,1.

The Periodic Table is linked to electron arrangement:

• The period tells you how many occupied electron shells the atom has.
• The group tells you the number of outer-shell electrons for the main groups.
• Elements in the same group have similar chemical reactions because they have the same number of outer-shell electrons.

Metals and non-metals

A metal is an element that usually conducts electricity and heat, is malleable, and forms positive ions. Metals are mostly found on the left and in the middle of the Periodic Table.

A non-metal is an element that usually does not conduct electricity, is often brittle if solid, and tends to gain or share electrons. Non-metals are mostly found on the right-hand side of the Periodic Table. Hydrogen is a non-metal even though it is placed on the left.

Metals usually have 1, 2 or 3 electrons in their outer shell, so they often react by losing electrons. Non-metals usually have 4, 5, 6 or 7 outer electrons, so they often react by gaining or sharing electrons.

Chemically, metal oxides are usually basic, while non-metal oxides are usually acidic.

Why atoms bond

Atoms often become more stable when they have a full outer shell of electrons. Group 0 elements, the noble gases, already have full outer shells, so they are very unreactive.

A chemical bond is a force of attraction between particles. It is not a little stick or a piece of matter.

Atoms can bond by transferring electrons or by sharing electrons.

Ionic bonding: transfer of electrons

An ion is a charged particle formed when an atom or group of atoms loses or gains electrons.

• Losing electrons makes a positive ion.
• Gaining electrons makes a negative ion.

An ionic bond is the strong electrostatic attraction between oppositely charged ions. Ionic bonding usually happens between metals and non-metals.

For example, in sodium chloride, sodium transfers one electron to chlorine. Sodium becomes Na⁺ and chlorine becomes Cl⁻. The formula NaCl shows the simplest ratio of ions, not a separate molecule.

A binary ionic substance is an ionic compound made from two different elements, such as sodium chloride, magnesium oxide or calcium chloride.

Dot-and-cross diagrams for ionic compounds

A dot-and-cross diagram shows outer-shell electrons using dots for one atom and crosses for another. For ionic compounds:

1. Show electrons being transferred from the metal atom to the non-metal atom.
2. Draw the final ions in square brackets.
3. Add the correct charges, such as Mg²⁺ or Cl⁻.
4. Show full outer shells on the negative ions.

Covalent bonding: sharing electrons

A covalent bond is a shared pair of electrons between non-metal atoms. The shared electrons are attracted to the nuclei of both atoms, holding the atoms together.

A molecule is a group of atoms held together by covalent bonds. Examples include H₂, Cl₂, H₂O and CO₂.

Comparing types of structure

Bonding is about forces between particles, but substances also have different structures, meaning different arrangements of atoms, ions or molecules.

Ionic compounds

Ionic compounds form a giant 3D lattice of alternating positive and negative ions. The ionic bonds act in all directions through the lattice.

Simple molecular substances

A simple molecular substance contains small separate molecules. There are strong covalent bonds within each molecule, but much weaker forces between molecules.

Giant covalent structures

A giant covalent structure is a huge network of atoms joined by covalent bonds. Examples include diamond, graphite and silicon dioxide. The covalent bonds continue throughout the structure.

Polymers

A polymer is a long-chain molecule made from many repeating units. The atoms in the chain are joined by covalent bonds. Polymer chains can be represented in 2D, but the real molecules are 3D and can be tangled or coiled.

Metals

In a metal, positive metal ions are arranged in a regular lattice. The outer electrons become delocalised, meaning they are free to move through the structure. A metallic bond is the electrostatic attraction between positive metal ions and delocalised electrons.

Limits of bonding models

Models help you think, but every model leaves something out.

Dot-and-cross diagrams are useful because they show electron transfer or sharing clearly. However, they do not show the real size of atoms, the 3D shape of molecules, or the movement of electrons.

Ball-and-stick models are useful for showing which atoms are connected and the approximate shape. However, bonds are not physical sticks, and the atoms are not usually shown at their true relative sizes.

2D diagrams of giant structures are useful on paper, but real ionic lattices, covalent networks and metal lattices are 3D.

Mendeleev and the modern Periodic Table

Mendeleev arranged elements mainly by atomic mass, but he also used chemical properties. He left gaps for undiscovered elements and predicted their properties.

The modern Periodic Table is arranged by atomic number. This improved the table because atomic number links directly to proton number and, in neutral atoms, electron number. That means the modern table lines up with electron arrangements and repeating chemical properties.

As atomic number increases, the number of protons increases by one each time. In neutral atoms, the number of electrons also increases by one, so the electron arrangement changes in a regular pattern. This is why elements in the same group have similar reactions.